This morning, as I closed my apartment door, I heard meowing outside. I got out onto the porch, and the tortoiseshell cat was standing at the end of the walk, meowing frantically. I crouched down, and it with starts and stops came up to me to be petted, still crying. It wasn't superthin, but bonier back than belly which usually... portends.
I went inside and she practically followed me in, got a bit of food, which she ate hungrily and so I gave her a whole bowl and left her eating on the porch, with Myrtle half in and half out of her cat window, watching warily.
Hopefully she's just an inveterate mooch and not yet another abandoned kitty.